The proposal is a continuation of the work of this laboratory on neurohormonal regulation of gastric activity. Three subprojects are proposed: 1. The relation between chemical structure and biological activity of peptides related to gastrin and cholecystokinin will be studied. The aim is to determine the exact structural requirements for resistance to inactivation by the liver, for selective action on pancreatic rather than gastric secretion, and for inhibition rather than stimulation of gastric secretion. 2. The relative potency of individual amino acids will be studied in respect to stimulation of acid secretion and of gastrin release. The aim is to determine whether the relative potencies of the amino acids are the same for the 3 sites of their action: oxyntic gland area, antrum, and intestine. 3. Inhibition of gastrin secretion by acidification of the duodenal bulb will be studied pharmacologically. The aim is to characterize the mediator of this neural reflex.